Ricochet Modem

The reality is that these devices work quite nicely on desktop systems as well as on laptops.
What's The Future?

Since I got my modem last fall, Metricom has offered two new products: a modem (without battery) for fixed stations and a newer, lighter weight version (8 ounce versus 13 ounce) of the portable modem. An even lighter version is being tested for palm-top computers. In addition, Ricochet has distributed a firmware upgrade that allows a roaming user to remain connected while moving from one wired-access-point coverage area to another.

The current technology operates in the 908-926MHz band, dividing it into 162 channels, each 160KHz wide. Each connection hops among these channels using spread spectrum technology. Modem radios provide a connection with performance comparable to a 28.8Kbps modem, but may burst higher. (That's why they recommend setting the serial port to 57.6Kbps.) Future models will use more channels and other bands to provide better coverage and higher speeds (128Kbps to 512Kbps).

What If You're Not In One of The “Blessed Cities”?

If you're traveling, you'll find service outside of these cities in airports such as Minneapolis-Saint Paul International Airport, Phoenix Sky Harbor Airport and La Guardia Airport in New York City.

Several schools, including Austin College, California Polytechnical Institute, George Washington University, Oregon State University, San Francisco State, Stanford University, UC Berkeley, UC Santa Cruz, University of Miami and University of Oregon, have established local service areas for their students, staff and faculty.

In addition to service provided in big cities, schools and airports, Metricom has teamed up with local utility companies to provide coverage in smaller towns—systems are being installed in Casper, Gillette, Laramie, Lander and Torrington in Wyoming and Scottsbluff, Gering, Kearney, Alliance and McCook in Nebraska. The nature of the Ricochet system allows Metricom to offer localized service wherever there are enough users to support it—I have visions of service in places like Durango, Colorado.

Remember the strange phone number? You can also enter the serial number of another modem radio. If it's nearby and not currently engaged, it will see a “RING” and the other user's program can answer the phone. In this way a peer-to-peer service is established between two modems.

Figure 2. Map of Puget Sound Coverage

There's another form of operation as well—called Star mode. In normal mode, the modem acts like a Hayes-compatible modem; in Star mode, the modem delivers packets into space (the original ether) and other modems see these packets and deal with them in a manner similar to an Ethernet card. This mode only works in modem-to-modem communication; however, the greater bit capacity makes it worth exploring. If you look in the Linux kernel configuration, you'll find support for this mode is already available. With this code compiled in, you and your co-workers and friends can have your own wireless LAN wherever you go. It's very convenient for conferences.

About The Laptop...

I finally broke down and bought one. Naturally, it runs Linux, and when I'm out of the office, I unplug the PCMCIA Ethernet card and turn on the Ricochet modem.

When the microcell radios are installed, they're configured with their latitude and longitude. One of the extra AT commands causes the modem to report the location and signal strength of up to ten nearby microcell radios. I've wandered around Seattle getting these reports and looking for microcell radios. I edited these reports into the tms file cited in the URL above—that's how I built the map.

Needless to say, I had to check out the roaming feature once I got my laptop. As I drove around town, I issued a ping command directed at my home system. Sometimes the latency was quite high, but it also seemed to go down when a second ping was issued. I suspect that routing tables are updated often enough that the system quickly finds the best route. Now I've got to check it while riding the ferry across Puget Sound.

Visit http://www.ricochet.net/ for more information.

Randolph Bentson has been programming since 1969—writing more tasking kernels in assembly code than he wants to admit. He has been working with Unix for nearly 16 years, and he's been enjoying and contributing to Linux for the last 3 years. Mr. Bentson is the author of Inside Linux, A Look at Operating System Development [1996, SSC]. He can be reached via e-mail at bentson@grieg.seaslug.org.